1. Field of the Invention
The present invention is related to touchscreens, and, more particularly, to a method of reducing noise associated with the touchscreen sensor.
2. Description of the Related Art
Most of the sensing systems used in touch screen applications have a lower sampling frequency compared to the surrounding noise. Single tone noise is very difficult to filter, especially if it has alias(es) in the low frequency region. Unfortunately single tone noise is everywhere, emanating from DC-DC converters, D-class amplifiers, and other switching components.
In the case of a sensor with a fixed sampling frequency, any single tone signal with frequency higher than ½ of the sampling frequency will have aliases. If the alias lays on the low frequency region, a simple average (low pass filter) will not help to suppress it.
Referring now to FIG. 1, a plot is shown of output data standard deviation versus noise frequency where the sampling period is 43 μS in this example. The sensor used to generate the plot shown in FIG. 1 is explained in greater detail below, and number of accumulation is 16×. Evidence of aliases that cannot be filtered by 16× averaging can be seen as peaks of standard deviation in the graph below in FIG. 1. The peaks are generally lower at higher frequency due to analog integration/low-pass filter applied in the sensor for each single conversion (before 16× averaging). It can be seen that although 16× averaging is used, there are frequencies where noise cannot be filtered.
What is desired is a noise reduction method for a touchscreen sensor so that the noise components related to sampling aliases can be minimized.